Elements of dam engineering

CHAPTER 1
Elements of Dam Engineering
Compiled by: Chala. H (MSc)
Jimma University
1

1.1. Introduction
▪ A hydraulic structure may be defined as any structure
which is designed to handle water in any way
▪ This includes the retention, conveyance, control,
regulation and dissipation of the energy of water
▪ Such water handling structures are required in many
fields of civil engineering
▪ The principal ones being water supply and
conservation, hydroelectric power, irrigation and
drainage, navigation, flood control, fish, wildlife
service’s and certain aspects of highway engineering
2

cont…
➢ Various equations, based on continuity, energy, and
momentum principles, may be used
➢ To calculate the most suitable length, width, shape,
elevation and orientation of the structure.
➢ The application of these basic principles to the
practical problem of the design of hydraulic structures
is called hydraulic design
➢ Designed and constructed for managing and utilizing
water resources to the best advantage of the human
being and environment
3

1.2. Classification of hydraulic structures
The principal hydraulic structures can be grouped under
the following three main categories :
✓Heading‐up and Control of water
✓Conveyance and Navigation works
✓Storage works

cont.….
Heading‐up and Control of water
➢ Their main purpose is to raise the upstream water
level to feed the canals/tunnel
➢ They also serve in controlling and regulating the rate of
flow and measurement of flow in the stream.
It comprise:
✓ weirs
✓ Barrages

Conveyance and Navigation works
➢ Structures which are used for transportation and to
transport the fluid it self.
Conveyance and Navigation works includes:
➢ Pipes
➢ Open Channel
➢ Pressure conduit
➢ Canal system
➢ Tunnels
➢ Sewer pipes and etc.

Storage Works
➢ structures which store water for future use.
➢ The stored water can be used when the natural flow
is less than the required demand. It comprises:
✓ Large dams and Tanks
10

reservoir
Dam axis
Spill way
Tail water

Classification of Hydraulic Structures by
function

1.3. Dam and reservoir structures
Dam structures
➢ Dams is a barrier across flowing water that obstructs,
directs or retards the flow, often creating a Reservoir.
➢ The blockage or storage formed, can be utilized for
various water resources development purpose
➢ The primary purpose of a dam is to provide safe
retention and storage of water.

Cont.…
The specific main functions /advantages of dams
and reservoirs are:
➢ Water storage for days, weeks, months, years, and over
years for power generation and sediment control.
➢ Water supply (municipal, industrial or community
water supply, irrigation purposes)
➢ Flood control
➢ Regulation/balancing of river flow (such as navigation,
low flow augmentation)
➢ Recreation
➢ Multipurpose

Cont.…
Construction of the dam have also disadvantage
The disadvantages includes
➢ Distraction of fauna and flora in the dam valley
➢ Interference in the landscape through dams and
reservoirs
➢ Change of ground water level
➢ Strong influence on sediment balance
➢ Change in water temperature and quality
➢ Influence on fish
➢ Displacement of people from reservoir site

Cont.….
Structure related to Dam includes
➢ Spillway
➢ Bottom outlet
➢ Intake
➢ Gate and valves
➢ Service road
➢ Reservoirs
➢ Bridge
➢ Energy dissipaters
➢ Diversion structures

Figure Schematic diagram of the dam components
1. Reservoir
2. Dam
3. Service Intake
4. Bottom outlet
5. Spill structure
6. Diversion structure
7. Service road

Reservoir
➢ A reservoir is a large, artificial lake created by
constructing a dam across a river
➢ Broadly speaking, any water pool or a lake may be
termed as a reservoir.
➢ However, the term reservoir in water resources
engineering is used in a restricted sense for a
comparatively large body of water stored on the
upstream of a dam constructed for this purpose
➢ Thus a dam and a reservoir exist together.
➢ The discharge in a river generally varies considerably
during different periods of a year
18

Cont.…
➢ If a reservoir serves only for one purpose, it is called a
single-purpose reservoir.
➢ On the other hand, if it serves for more than one
purpose, it is termed a multipurpose reservoir.
Reservoir
Dam
Upstream
Downstream

Cont.…
Depending upon the purpose served by a given reservoir,
the reservoir may be broadly classified
➢ Storage or conservation reservoir: Those are reservoirs
which retain excess supplies during period of peak flow
and can release gradually during low flows or when the
need arises.
➢ Flood control reservoirs:- store a portion of flood
flows in such a way to minimize the flood peaks at the
area to be protected downstream.
➢ Multipurpose reservoirs:-are those reservoirs which are
planned and constructed to serve not only one purpose
but various purpose together.

Purposes of storage works
1. Irrigation
2. Hydro-electric power generation
3. Control of destructive floods
4. Low water regulation for navigation
5. Domestic and industrial water supply
6. Recreation
7. Preservation and breeding of useful aquatic life, etc.

Storage Components
The reservoir volume is subdivided in to the following parts, when seen in its
vertical section
Dead storage
Minimum pool level
Normal pool level
Live storage
Maximum pool level
Spillway
Outlet
Spillway crest
River bed
Surcharge storage

Cont.….
Dead storage: The portion of the storage capacity from
which water is not normally withdrawn, in accordance
with operating agreements. It is the storage that lies below
the invert of the lowest outlet and thus cannot be evacuated
by gravity.
Live storage: The volume of storage in which water will be
taken for demand, using service outlet. It is the portion of
the storage capacity in which water will normally be stored
and withdrawn for beneficial uses.
Surcharge storage: This is required as a reserve between
full reservoir Level and the Maximum Water level to contain
the peaks of floods

Cont.…
➢ Bottom outlet: The outlet which is mainly used to
discharge the water from the reservoir for the
environmental flow and for flushing of sediments.
Some times it is also opened to discharge when there is
flooding on the upstream.
➢ Service outlet: It is used to discharge the required
amount of water for downstream beneficiaries as their
need.

Basic terms
▪ Yield from a reservoir: is the volume of water which can
be withdrawn from a reservoir in a specified period of time.
▪ Safe yield (Firm yield): is the maximum quantity of water
which can be supplied from a reservoir in a specified period
of time during a critical dry year.
▪ Secondary yield: is the quantity of water which is
available during the period of high flow in the rivers when
the yield is more than the safe yield.
▪ Average yield: is the arithmetic average of the firm yield
and the secondary yield over a long period of time.
▪ Design yield: is the yield adopted in the design of a
reservoir. The design yield is usually fixed after considering
the urgency of the water needs and the amount of risk
involved.

1.4. Storage Capacity of the dam
➢ Reservoirs important function is to store water during
floods and to release it later, i.e., storage capacity is its
most important characteristics.
➢ The available storage capacity of a reservoir depends upon
the topography of the site and the height of dam.
➢ To determine the available storage capacity of a reservoir
up to a certain level of water, engineering surveys are
usually conducted.
➢ The storage capacity and the water spread area at different
elevations can be determined from the contour map
➢ A contour plan of the area is prepared to a scale of 1 cm =
100 m or 150 m with a contour interval of 1 to 3 m,
depending upon the size of the reservoir

Cont.…
Area-Elevation Curve
➢ From the contour plan, the water spread area of the
reservoir at any elevation is determined by measuring the
area enclosed by the corresponding contour.
➢ The storage capacity of the reservoir at any elevation is
determined from the water spread area at various
elevations.
➢ Generally, a planimeter is used for measuring the area.
An elevation-area curve is then drawn between the
surface area as abscissa (x-axis) and the elevation as
ordinate (y-axis).
➢ Generally, the volume is calculated in Mm3.
30

…cont…
Elevation area curve

Cont.….
Reservoir Storage Capacity determination
The following formulae are commonly used to determine
the storage capacity
▪ Trapezoidal formula: According to the trapezoidal
formula, the storage volume between two successive
contours of areas A1 and A2 is given by
where h is the contour interval.
Therefore the total storage
volume V is
where n is the total number of areas.

Cone formula: According to the cone formula, the
storage volume between two successive contours of
areas A1 and A2 is given by
Cont.…

Prismoidal Formula: According to the prismoidal
formula, the storage volume between three successive
contours is given by
▪ The prismoidal formula is applicable only when
there are odd numbers of areas (i.e. n should be an
odd number).
▪ In the case of even number of areas, the volume up to the
second last area is determined by the prismoidal formula,
and that of the last segment is determined by the trapezoidal
formula.
Cont.…

Example: 1) A reservoir has the following areas enclosed by contours
at various elevation
Elevation(m) 200 220 240 260 280 300
Area (km2) 150 175 210 270 320 400
Determine the volume of the reservoir Use
a. Cone method
b. Trapezoidal
c. prismoidal formula

1.5. Classification of dams
Dam: Is a barrier across flowing water/river that
obstructs, directs or retards the flow of water, often
creating a Reservoir.
There are various ways of classification of dams. Some of
them are :
i. Classification based on function served
ii. Classification based on hydraulic design
iii. Classification based on material used
iv. Classification based on rigidity

i. Classification according to use
➢ Storage dam: ‐ It is a common type of dam constructed to impound
surplus flood water for use in dry period. It may be constructed of a
wide variety of materials such as concrete, stone, earth and rock‐fill.
➢ Diversion dam: ‐ It is a dam of smaller height constructed to raise
water level of the river and not for storage or equalization of flow.
➢ Detention dam: ‐ It is constructed to store water during floods and
release it gradually at a safe rate when the flood recedes.
➢ Debris dams:‐ is constructed to retain debris such as sand, gravel and
floating woods and the water that passes over the dam is relatively
clear.
➢ Coffer dams: ‐ it is constructed to divert the water during period of
construction. It is temporary dams constructed for facilitating
construction. It is constructed mainly on upstream but also on
downstream depending the slope of the river at the site
37

ii. Classification according to hydraulic design
a. Overflow dam: They are designed to pass the surplus
water over their crest. They must be made of materials
which will not be eroded by such discharges .E.g. concrete,
masonry etc…
b. Non‐overflow dam: They are those which are not
designed to be overtopped. This type of design extends
the choice of materials to include earth fill and rock fill
dams.

iii. Classification according to construction
materials used
a) Masonry dams: ‐ made of masonry
b) Concrete dams:‐ made of concrete
c) Earth dams:‐ made of earthen materials
d) Rock‐fill dams:‐ made of rock
e) Timber dams: ‐made of timber
f) Steel dams: ‐ made of steel

iv. Classification based on rigidity of
construction material
a. Rigid dams
➢ It is constructed from rigid materials such as masonry,
concrete, steel or timber
➢ These dams deflect and deform very little when
subjected to water pressure and other forces.
b. Non‐rigid dams
➢ It is constructed from non‐rigid materials such as earth
fill and/or rock‐fill.
➢ There are relatively large settlements and deformations
in non‐rigid dam

1.6. Characteristics of dams
Characteristics of concrete dams Gravity dam
➢ It is constructed from concrete materials
➢ Its stability is based on its own weight
➢ The gravity dam section profile is essentially triangular to
ensure stability and to avoid over-stressing of the dam or its
foundation.
➢ Concrete gravity dams could be straight or curved in plan
➢ It is suited to wider valleys( relative to arch dam), provided
that excavation to rock is less than about 5m
➢ It needs sound rock foundation
➢ Requires imported cement

reservoir
Dam axis
Spill way
Tail water
43

Advantageous of Concrete Dams
➢ Good for narrow valleys.
➢ it also suitable for wide valleys.
➢ Smaller construction fill volume (compared to
embankments)
➢ Construction can takes place irrespective of weather
condition
➢ Smaller free board (1-1.5m)
➢ Can be constructed as overflow dam, i.e. can
accommodate spillway, hence cost for separate
spillway reduced.
➢ Outlet pipe work, valves and other ancillary works can
be provided within the body of the dam

Disadvantageous of Concrete Dams
➢Sensitive to settlement, demanding sound rock
foundation
➢Sensitive to earth quake
➢Depending on the dam type, it’s construction is
demanding (technical personnel, machine
requirement, form work)
➢Bad interference in to natural landscape
➢Completed unit costs are much higher than
embankment fills, which seldom balanced by the
much lower volumes of concrete required in a dam
of certain height.

Characteristics of Arch dam
➢ Arch dam is dam with upstream curvature which
constructed from concrete materials.
➢ Stability is obtained by a combination of arch and
gravity action.
➢ Utilizes the strength of an arch to resist loads placed
upon it by 'arch action‘.
➢ The strength of the rock mass at the abutments and
immediately down valley of the dam must strong
enough.
➢ It is suited to narrow gorges/ V-shape valley.
➢ High abutment loading.
➢ 50-85% concrete saved relative to gravity dams.

Cont.…
➢ The foundations and abutments must be competent not
only to support the dead weight of the dam on the
foundation but also the forces that are directed into the
abutments because of arch action in response to the
forces acting on the dam.
➢ Arch dam can be classified as
✓ Single‐curvature arch dams
✓ Double –curvature arch dams
✓ Arch‐gravity dams

Types of diversion head Work
1. Diversion head work:
✓ Divert the required supply of water from the source
channel to the off taking channel.
✓ The water level in the source channel is raised to the
required level so as to divert the required supplies into
the off taking channel.
✓ The head work must have an arrangement for controlling
the sediment entry into the channel off taking from a
river.(work for head)
1. Storage head work: Besides fulfilling all the
requirements of diversion head work, store excess water
when available and release it during periods when
demand exceeds supplies.(work for storage)

Characteristics of buttress dam
➢ Buttress dams require between one thirds and half of
the concrete required for a gravity section, hence
making it more economical for dams over 14m.
➢ Concrete saved 30-60% relative to gravity dams.
➢ Threat of deterioration of concrete from the
impounded water is more likely than from a thick
gravity section.
➢ There is also an elimination of a good deal of uplift
pressure.
➢ create higher contact stress, needs sound rock.

Gravity Dam
54
Load Resisting Mechanism of concrete dams

Gravity Dam
Arch Dam
55
Load Resisting Mechanism of Arch dams

Gravity Dam
Arch Dam
Buttress Dam 56
Load Resisting Mechanism of buttress dams

Characteristics Embankment dams
➢ Any dam constructed of naturally excavated materials placed without
addition of binding material other than those inherent in the natural
material.
➢ It is suited to either rock or compressible soil foundation and wide valley.
➢ It can accept limited differential settlements.
➢ Embankment dams are usually referred to be types such as Earth fill,
Rock fill, and Hydraulic fill.
➢ Earth fill dam: An embankment dam constructed primarily of compacted
earth in either homogeneous or zoned areas containing more than 50% of
earth.
➢ Rock fill dam : The term rock fill dam usually represents dams that
contain more than 50% compacted or damped pervious rock
➢ Hydraulic Fill dam: An embankment dam constructed of earth, sand,
gravel or rock generally from dredged material conveyed to the site of
placement by suspension in flowing water.

Cont…
05-Nov-2019 Hydraulic structure I 62

1.7. Phases of Engineering activity for site
investigation and construction of dam
What are the criteria to start huge project like dam?

Phases of Engineering activity for site
investigation and construction of dam
Dams must be planned, designed, and constructed to operate
efficiently
a. Prior (preceding) to construction
Reconnaissance
➢ The main purpose of such investigations is to screen out the
poorer(minor) alternatives.
➢ To decide the types and amounts of more expensive and
time-consuming data (such as stream flow records,
topographic mapping, and so on) which need to be
collected for making feasibility investigations of the
remaining selectable alternatives.

Cont.…
Reconnaissance
✓Field trips and reconnaissance of dam site, reservoir
area and downstream area.
✓Collection and evaluation of existing data
Like:-
▪ Socio-economic
▪ Political
▪ Environmental and
▪ Physical data of the catchment

Cont.….
Hydrologic data
➢ Collection and analysis of stream flow (mean monthly
flow ) and precipitation record
➢ Annual sediment load
➢ Maximum observed flood level in the river
➢ Report on damage caused by flooding
➢ approximate maximum and minimum daily water
demand and Assessment of available yield
➢ A flood occurring once in 100 years or less may cause
enormous damage. Therefore, stream gauging records
of 10, 20 or 30 years required based on size of dam

Cont.….
Meteorological Data
➢ Average monthly temperature
➢ Average monthly rainfall
➢ Maximum recorded storm intensities
➢ Annual rate of evaporation
➢ Data collection like, precipitation, evaporation,
infiltration and percolation condition of the site.
Geological Data:
➢ Geologic map of location site
➢ Nature and type of rock of foundation and
abutments(support)
➢ Geologic condition of dam and Spillway site

Cont.….
Earthquake information
➢ Historical seismic information
Construction material type
➢ Possible quarry sites for construction materials should
be identified with in a reasonable distance from the
construction site
Miscellaneous data
➢ Erosion condition in the catchment area,
➢ Transportation
➢ Existing facilities and rates
➢ Local labor

Cont.…
Preliminary and Feasibility studies
➢ Detailed investigation of site conditions
➢ The data collected is used for preliminary design and
economic analysis
➢ Final selection of dam type, main dimensions, dam site and
layout, costs and etc.
Final investigation and Basic design
➢ Completion of detailed site investigations
➢ Detailed design, cost estimates, construction schedules,
➢ Development of:
✓ Basic report
✓Tender documents for the construction of civil works
(drawings, specifications, general and special contract
conditions)

Cont.…
b. During Construction
➢ Detailed (design and construction execution)
➢ Stepwise detail of dam design ahead of construction.
➢ Site investigations oriented towards construction.
Construction activities
➢ Preliminary installation of contractor at site
➢ Construction of required infrastructure (road, camp, plant,
water and energy supply, etc.)
➢ River diversion
➢ Construction of dam and associated structures
Commissioning and operation manuals
➢ Preparation of manuals for testing, monitoring and operation of
equipment.
➢ Maintenance and service

Cont.…
c. After construction
Monitoring, surveillance, operation
➢ Reading of instruments, evaluation of results
➢ Planning of operation
➢ Execution of operation activities

1.7.1 Factors governing Selection of site for dam
As dams require huge amount of investment cost, care should
be taken while selecting the suitable site.
➢ The following factors shall be considered when selecting
the site of a dam.
Topography
✓ As much as possible, the dam should be located where the
river has narrow gorge which opens out upstream to create
a large reservoir .
Suitable foundation:
✓ Suitable foundation is necessary for dam site
✓ The existence of joint patterns in an abutment (their
orientation, inclination and infilling) affect the stability of
the dam.

Cont.…
✓ Rock foundation: ‐Any type of dam can be constructed
on good rock foundation.
• Such foundations have high bearing capacity and
resistant to erosion and percolation and are ideal for all
types of dams
✓ Gravel and coarse sand foundation: ‐ such foundations
have low bearing capacity and earth and rock fill dams
are suitable
• As there may be high seepage, cut‐off may be provided
✓ Fine sand and silt foundations: ‐ it is suitable only for
earth and low height concrete dams.
• As there is settlement, piping, seepage erosion at the d/s
toe and liquefaction failure, measures should be done.

Cont.…
✓ Clay foundation: ‐ it is not suitable for construction of
dam. These foundations have very low bearing
capacity. The settlement is quite large and hence it
needs foundation treatment before dam construction.
Availability of Technical skills
• At many sites neither skilled contractor nor artisans are
available.
• This can preclude the adoption of dams that require
intricate framework or very high quality concrete. The
embankment type dams or a masonry gravity dam may
then prove most suitable.

Cont.…
Cost effectiveness
• Site conditions naturally influence the cost of various
types of dam.
• Expenditure for operation and maintenance (including
replacement) of the project.
• The capital cost includes the costs of planning,
investigations, designs, and construction besides the cost of
acquiring rights to the use of water, litigations, and
rehabilitation of the affected people.
• The benefits likely to be received from a water resource
project are have to be known.

Cont.…
• For a very high dam, for instance, the cost may be high.
For concrete dams costs for external spillway can be saved
in case of spillway located at the dam crest. However,
standard prices of earth and rock fill materials in the past
have not risen as much as those of mass concrete dams.
Availability of materials
• The cost of transport of material to the site has to be
considered, for different dam options.

Cont.…
Environmental and related issues
• Environment is best defined as all external conditions which
affect the existence of all living beings.
• Different living beings affect one another, and the environmental
requirements of different living beings are interrelated
• The environmental, economic and other socio-political issues
associated with reservoir development must in all instances be
acknowledged at the outset and fully addressed thereafter.
• This is especially important in the case of the larger high-profile
projects.
• Especially on cross boundary river

Cont.…
Hydrology
• The possibility of inundation by floods during construction may
favor a concrete dam.
• Where high flow occurs, an embankment dam will require
special and detailed protection; whereas concrete dam could be
overtopped with little damage.

summarizes dam type characteristics in the
context of selection

1.7.2. Dam Site Assessment and Investigation
• Parties directly involved in engineering of dam activities
are:
- Owner - public or private organization
o Provision of financial resources
o Establishment of general objectives
- Consulting Engineer
o Planning and design
o Construction supervision
- Contractor for civil works
o Construction of required infrastructure
o Construction of civil works
• Assembly of equipment

Spillway site selection
➢ A good site for spillway should exist at or near the dam
site.
➢ Availability of construction materials: dam requires
large amount of construction materials and hence, there
should be sufficient amount of construction material
with high quality and quantity at nearby to reduce
transportation cost.
➢ Water tightness of reservoir: the bed and side of the
reservoir should be water tight to reduce seepage loss
➢ Submergence area: the area submerged on the
upstream of the dam should be small and should not
submerge costly land and property

Cont.….
➢Accessibility: ‐ the site should be easily accessible
for transportation of construction material. If there
was no road to the site, access roads should be
constructed prior the construction.
➢Sediment rate: ‐ the incoming water should be
relatively free from sediment in order to increase the
life span of the reservoir
➢Minimum overall cost: ‐ it should be relatively
minimum cost including maintenance and operation

1.7.3. Foundation of dams and their treatment
➢ Foundation treatment is a technique of improving the
foundation property for the construction of high
concrete structures.
➢ For convenience, foundation may be classified as rock,
coarse‐grained materials and fine‐grained materials
foundations
➢ The commonly adopted foundation treatments are:
✓ Surface preparation and
✓ Foundation grouting.

Cont.…
Surface preparation
✓ Removing the entire loose soil till sound bedrock is exposed
without damaging the underlying rock.
✓ If faults, seams or shattered rock zones are detected in the
exploratory geological investigations, special steps and remedies
must be taken to ensure their removal.
Foundation Grouting
✓ Pressure grouting is the process of injecting suitable
cementations’ slurries or similar materials into inaccessible places,
such as the underlying formation of foundation of dams for the
purpose of sealing seams, cracks and fissures or filling voids and
also used to improve the strength and elastic properties of the
material into which it is injected

Cont.….
In general, grouting for foundation treatment may be classified as
Consolidation or Area Grouting and Curtain grouting
a. Consolidation or Area Grouting
➢ The objectives of consolidation grouting are to strengthen the
rock, to stop water passage through the disintegrated rock and to
increase bearing strength of the strata.
➢ It is done in an area, usually the entire dam foundation, by drilling
shallow holes (3 to 15 m deep) on a grid pattern at a spacing of 5 to
30 m.
➢ This comparatively shallow and low pressure grouting is done for
the general consolidation of the foundation before concreting of
the dam section and is followed by high pressure grouting after
some concreting of the dam has taken place.
➢ It also helps in checking the leakage of high‐pressure grout
used later in the curtain grouting

Cont.…
Curtain grouting
➢ Curtain grouting (or high pressure grouting) is done to
form a deep curtain wall or an impervious barrier in the
foundation just down‐stream of the heel of the dam.
➢ It is high pressure grouting is required and relatively
deeper holes are drilled near the heel of the dam.
➢ It helps in reducing seepage through the foundation
and reduces the uplift pressure.
➢ Holes required for curtain grouting are quite deep.
The holes are usually vertical but some times it could
be inclined. Inclined holes are required to intercept the
inclined joints in the rock to develop an effective grout
curtain.

Elements of dam engineering

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